Long-day rosette plants, such as lettuce and spinach, grow as rosettes (no stems) during the short days of winter, and produce stems and flowers (bolting) during late spring and summer when the days are long. Previous work has shown that production of gibberellins, a class of plant hormones, is stimulated by long days. The resulting increase in gibberellin level causes the stems to elongate. The objective of the proposed research is to investigate how the daylength regulates gibberellin biosynthesis at the biochemical and molecular levels. Present work is focused on the regulation and expression of gibberellin 20-oxidase, the multifunctional enzyme that catalyzes three successive steps in the gibberellin biosynthetic pathway: GA53?GA44?GA19?GA20. The GA 20-oxidase encoded by P16 of pumpkin endosperm converts GAs predominantly to inactive C-20 carboxylic acids. We introduced the P16 gene into Arabidopsis to see whether this would result in decreased levels of active GAs and, as a result, in decreased stem growth. GC-MS (SIM) was used to characterize GC metabolites. These results indicated there is a high degree of feedback inhibition in GA biosynthase.